1. Field of the Invention
The present invention relates to a cathode ray tube (CRT), and more particularly, to a CRT with enhanced brightness, contrast and color.
2. Description of the Related Art
A CRT is a type of display in which an image is displayed when red (R), blue (B) and green (G) phosphors, which are located in a black matrix (BX) in a dotted or striped pattern, emit colors of light in response to being hit by electron beams emanating from an electron gun.
FIG. 1 is a schematic view of a phosphor screen in a CRT. Light visible to viewers falls into two categories with respect to the light source; one type of light 15 emitted light 1 from phosphors being hit with electrons, and the other type of light is due to the back reflection of ambient light by the CRT. Also, the light reflected by the CRT includes light 2 reflected on the external surface of glass panel in the CRT and light 3 transmitted through the glass and reflected back by phosphors. The lights 1 emitted by the phosphors, having peaks of a predetermined wavelength, is used to display an image in a desired color by the combination of the emitted light 1. Because the ambient light has a continuous wavelength in a visible range, the wavelengths of the reflected ambient light are different to those of the light emitted from the phosphors, thereby lowering the contrast of a screen.
FIG. 2 shows the light emission spectrum of a P22 series phosphor. ZnS:Ag phosphor 1 has a major peak at 450 nm. ZnS:Au,Cu,Al phosphor 2 has a major emission peak at 540 nm with a width of about 130 nm, and Y2O2S:Eu phosphor 3 has a sharp major emission peak at 630 nm. However, the reflected ambient ray 2 and 3 of FIG. 1 is from a white light source having continuous light spectrum over the entire visible range, so that they include light at the wavelengths between the emission peaks of the phosphors of FIG. 2, unlike the emission of phosphors. Also, the peaks of B and G phosphors, which are broad, overlap each other near 490 nm and 580 nm, so that the contrast and color purity are deteriorated. In order to enhance the color purity in a CRT, the emission and reflection of light near 490 nm and 580 nm should be reduced.
A conventional method for improving the contrast of a CRT screen is to reduce the transmittance of glass in a CRT panel. The glass used in a CRT panel includes a transparent glass having a light transmittance of 75% or more with respect to visible light, a semi-tinted glass having a transmittance of 50xcx9c60% or more, and a dark tinted glass having a transmittance of 40xcx9c50%. FIG. 3 shows the transmittance distribution of these glasses. In FIG. 3, the transparent glass (a), the semi-tinted glass (c), the dark tinted glass (d) show an essentially constant transmittance over the entire visible range. If the transmittance of glass is lowered in order to enhance contrast, brightness is decreased. Also, if the transmittance is increased, the contrast is lowered while the brightness is enhanced. Thus, both brightness and contrast cannot be enhanced at the same time by adjusting the transmittance.
To solve this problem, technique of adding neodymium (Nd) to glass has been suggested. In FIG. 3, a curve b represents the transmittance distribution of glass containing Nd. The Nd-glass shows a selective transmittance, by having a major absorption peak near 570xcx9c590 nm and a minor absorption peak near 520xcx9c530 nm. These two absorption bands lie between the light emission regions of to the phosphors, so that the Nd-glass can absorb the light other than the light emission regions without changing the transmittance at the light emission regions of the phosphors, thereby enhancing both brightness and contrast. Also, since the Nd-glass can absorb light near 580 nm, which is very critical in improving the contrast and the color purity, the color purity can be also enhanced.
However, while the Nd-glass can improve the brightness and contrast characteristics, the light blue color of a CRT adopting the Nd-glass may be unattractive to viewers, unlike a black-colored CRT adopting a semi-tinted or dark tinted glass. Also, the reflectivity of a CRT adopting the Nd-glass is not low enough.
It is an object of the present invention to provide a cathode ray tube (CRT) having a high brightness and enhanced contrast and color reproduction range characteristics.
Accordingly, to achieve the above object, the present invention, as embodied and broadly defined herein, provides a cathode ray tube (CRT) having a phosphor screen in which red, green and blue phosphors are located in a black matrix in a dotted or striped pattern, at the inside of a panel, the CRT comprising a low-reflective coating layer at the outside of the panel, the low-reflective coating layer having a reflectivity of 2.0% or less near 480xcx9c600 nm, wherein the panel is formed of glass containing neodymium (Nd) of 0.1xcx9c1.5 wt % and praseodymium (Pr) of 0.1xcx9c0.7 wt %.
Preferably, the panel further comprises nickel (Ni) and cobalt (Co), and the weight ratio of Ni and Co is greater than 15:1. The low reflective coating layer may comprises: a first coating layer formed of silica; and a second coating layer formed of a material having a refractive index higher than that of silica. Preferably, the material for the second coating layer is selected from the group consisting of indium tin oxide (ITO), antimony tin oxide (ATO) and silver (Ag).
Preferably, the red phosphor is Y2O2S:Eu, the green phosphor is ZnS:Au,Cu,Al or ZnS:Cu,Al, and the blue phosphor is ZnS:Ag. Also, the red, green and blue phosphors may be coated with red, green and blue pigments, respectively. Preferably, the amount of pigment coated on each phosphor is adjusted such that the reflectivity of a short wavelength is 5% or more lower than that of a long wavelength. Also, any pigment which is common in the art, may be used.